Additive concentrate

ABSTRACT

An additive concentrate comprises a lubricity enhancer dissolved in a solvent. The lubricity enhancer comprises from 10 to 90% by weight of the concentrate and is a salt formed by the reaction of a carboxylic acid with di-n-butylamine or tri-n-butylamine. The concentrates have excellent low temperature stability.

This invention relates to additive concentrates, and to their use to improve the characteristics of fuel oils, especially middle distillate fuels such as diesel fuels, kerosene and jet fuel.

Environmental concerns have led to a need for fuels with reduced sulphur content, especially diesel fuel, heating oil and kerosene. However, the refining processes that produce fuels with low sulphur contents also result in a product of lower viscosity and a lower content of other components in the fuel that contribute to its lubricity, for example, polycyclic aromatics and polar compounds. Furthermore, sulphur-containing compounds in general are regarded as providing some anti-wear properties and a result of the reduction in their proportions, together with the reduction in proportions of other components providing lubricity, has been an increase in the number of reported problems in fuel pumps in diesel engines. The problems are caused by wear in, for example, cam plates, plungers, rollers, spindles and drive shafts, which may result in sudden pump failures relatively early in the life of the engine.

The problems may be expected to become worse in future because, in order to meet stricter requirements on exhaust emissions generally, higher pressure fuel systems, including in-line pumps, rotary pumps, common-rail pumps and unit injector systems, are being introduced, these being expected to have more stringent lubricity requirements than present equipment, at the same time as lower sulphur levels in fuels become more widely required.

Historically, the typical sulphur content in a diesel fuel was below 0.5% by weight. In Europe maximum sulphur levels have been reduced from 0.20% to 0.05% and in Sweden grades of fuel with levels below 0.005% (Class 2) and 0.001% (Class 1) are in use. A fuel oil composition with a sulphur level below 0.05% by weight is referred to herein as a low-sulphur fuel.

Such low-sulphur fuels may contain an additive to enhance their lubricity. These additives are of several types. In WO 94/17160, there is disclosed a low sulphur fuel comprising a carboxylic acid ester to enhance lubricity, more especially an ester in which the acid moiety contains from 2 to 50 carbon atoms and the alcohol moiety contains one or more carbon atoms. In U.S. Pat. No. 3273981, a mixture of a dimer acid, for example, the dimer of linoleic acid, and a partially esterified polyhydric alcohol is described for the same purpose. In U.S. Pat. No. 3,287,273, the use of an optionally hydrogenated dimer acid glycol ester is described. Other materials used as lubricity enhancers, or anti-wear agents as they are also termed, include a sulphurized dioleyl norbornene ester (EP-A-99595), castor oil (U.S. Pat. No. 4,375,360 and EP-A-605857) and, in methanol-containing fuels, a variety of alcohols and acids having from 6 to 30 carbon atoms, acid and alcohol ethoxylates, mono- and di-esters, polyol esters, and olefin-carboxylic acid copolymers and vinyl alcohol polymers (also U.S. Pat. No. 4,375,360).

Such additives are commonly provided in the form of additive concentrates where the active species is dissolved in a solvent, optionally together with other additives. Although largely effective as lubricity additives, it has been found that certain carboxylic acids and carboxylic acid esters, as well as other known lubricity additives, have the disadvantage of poor solubility at low temperature. This poor solubility is found in relation to the solvents used to provide additive concentrates (normally hydrocarbon based), and also in relation to the fuels into which the additive is added. This places a limitation on the storage and use of certain known lubricity additive concentrates at low ambient temperatures.

EP 0 798 364 A1 describes the use of a salt formed by the reaction between a carboxylic acid and an aliphatic amine to improve inter alia, the lubricity of a diesel fuel. The amines used have hydrocarbyl groups of between 2 and 50 carbon atoms, preferably between 8 and 20 carbon atoms, with amines such as oleyl amine being exemplified. There is no discussion of the low temperature behaviour of the salts. U.S. Pat. No. 6,277,158 describes a concentrate containing n-butlyamine oleate as a friction modifier for addition to motor gasoline. The authors state that despite substantial research investigating a wide range of friction modifier compounds, n-butylamine oleate was one of only two compounds which gave the desired stability over a temperature range, and effectiveness when used in combination with a detergent. The concentrate containing n-butylamine oleate is shown to be stable at −20° C. for extended periods although there is no indication of the amount of compound present in the concentrate.

US2002/0095858 relates to fuel oil compositions containing an additive formed by the reaction of a mono- or dicarboxylic acid of 6 to 50 carbon atoms with an amine having at least one branched alkyl substituent. These additives are shown to be effective lubricity enhancers for the fuel. Data are presented showing that the additives per se have good low temperature properties (e.g. pour point) and are stable when held at −25° C. for 3 days. To demonstrate an advantageous property of the additives, they are compared to a similar species derived from an amine with linear rather than branched alkyl groups, namely oleic acid neutralised with tri-n-butylamine. The data show that this comparative compound had poorer low temperature properties and crystallised after 3 days at −25° C. There are no data relating to any additive concentrates i.e. compound dissolved in a carrier fluid or solvent.

US 2002/0014034 describes the use of additives to improve the lubricity of a fuel oil. A suitable additive may be formed by the reaction of N, N-dibutylamine with an acid mixture consisting of 70% fatty acids and 30% resin-based acids. This additive is shown to be soluble in diesel fuel in an amount of 5% by weight. HFRR data are presented, but there is no discussion of the low temperature properties of the additives.

The present invention is based on the discovery that additive concentrates containing certain salts derived from certain aliphatic amines have greatly improved low temperature solubility when compared to closely related salts. The salts are effective lubricity enhancers for low sulphur content fuels. It is also an advantage that the salts used in the invention have relatively high flash points. This leads to benefits in terms of the ease of, and hazards associated with, their handling and transportation.

Thus in accordance with a first aspect, the present invention provides an additive concentrate comprising a lubricity enhancer dissolved in a solvent; wherein the lubricity enhancer comprises from 10 to 90% by weight of the concentrate; wherein the lubricity enhancer comprises a salt formed by the reaction of a carboxylic acid with di-n-butylamine or tri-n-butylamine; and wherein the concentrate remains clear and bright for at least 14 days when held at −30° C.

Preferably, the lubricity enhancer comprises from 10 to 80%, more preferably 20 to 80%, for example 33 to 75% by weight of the concentrate and the concentrate remains clear and bright for at least 14 days when held at −30° C.

In a particularly preferred embodiment, the lubricity enhancer comprises from 33 to 50% by weight of the concentrate, and the concentrate remains clear and bright for at least 14 days when held at −40° C.

In a further preferred embodiment, the lubricity enhancer comprises from 33 to 50% by weight of the concentrate, and the concentrate remains clear and bright for at least 28 days when held at −30° C.

In this specification, the use of the term ‘salt’ to describe the product formed by the reaction of the carboxylic acid and the amine should not be taken to mean that the reaction necessarily forms a pure salt. It is presently believed that the reaction does form a salt and thus that the reaction product contains such as salt however, due to the complexity of the reaction, it is likely that other species will also be present. The term ‘salt’ should thus be taken to include not only the pure salt species, but also the mixture of species formed during the reaction of the carboxylic acid and the amine.

The term ‘clear and bright’ used in this specification to describe the concentrate will be understood by those skilled in the art. It is a visual measurement and should be taken to mean that the concentrate contains essentially no solid, suspended particles and, to the naked eye, is completely free from any haze or phase separation.

It has been found that concentrates containing salts formed from di-n-butylamine or tri-n-butylamine display particularly good low temperature properties. This allows concentrates containing high amounts of active ingredient to be stored in conditions of low ambient temperature without the need for expensive and complex heated storage facilities. The applicants were surprised to find that salts prepared from both shorter-chain, linear di-amines e.g. di-n-ethylamine and from longer-chain, linear di-amines e.g. dihexylamine, were less effective. It was also surprising to note that even the corresponding salt prepared using mono-n-butylamine was considerably less effective.

Conveniently, either di-n-butylamine or tri-n-butylamine is used although mixtures of the two may also be used if desired. Most preferably, the salt is formed by the reaction with di-n-butylamine.

As carboxylic acid, those corresponding to the formula [R′(COOH)_(x)]_(y), where each R′ is independently a hydrocarbon group of between 2 and 45 carbon atoms, and x is an integer between 1 and 4, are suitable. Preferably, R′ is a hydrocarbon group of 8 to 24 carbon atoms, more preferably, 12 to 20 carbon atoms. Preferably, x is 1 or 2, more preferably, x is 1. Preferably, y is 1, in which case the acid has a single R′ group. Alternatively, the acid may be a dimer, trimer or higher oligomer acid, in which case y will be greater than 1 for example 2, 3 or 4 or more. R′ is suitably an alkyl or alkenyl group which may be linear or branched. Examples of carboxylic acids which may be used in the present invention include: lauric acid, myristic acid, palmitic acid, stearic acid, isostearic acid, neodecanoic acid, arachic acid, behenic acid, lignoceric acid, cerotic acid, montanic acid, melissic acid, caproleic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, coconut oil fatty acid, soy bean fatty acid, tall oil fatty acid, sunflower oil fatty acid, fish oil fatty acid, rapeseed oil fatty acid, tallow oil fatty acid and palm oil fatty acid. Mixtures of two or more acids in any proportion are also suitable. Also suitable are the anhydrides of carboxylic acids, their derivatives and mixtures thereof. In a preferred embodiment, the carboxylic acid comprises tall oil fatty acid (TOFA). It has been found that TOFA with a saturate content of less than 5% by weight is especially suitable. As is known in the art, TOFA contains small but variable amounts of rosin acids and isomers thereof. Preferably, TOFA with an abietic acid content of less than 5% by weight, for example, less than 2% by weight, is used.

In another preferred embodiment, the carboxylic acid comprises rapeseed oil fatty acid.

In another preferred embodiment, the carboxylic acid comprises soy bean fatty acid.

In another preferred embodiment, the carboxylic acid comprises sunflower oil fatty acid.

Also suitable are aromatic carboxylic acids and their alkyl derivatives as well as aromatic hydroxy acids and their alkyl derivatives. Illustrative examples include benzoic acid, salicylic acid and acids derived from such species.

Preferably, the carboxylic acid has an iodine value of at least 80g/100 g, more preferably at least 100 g/100 g, for example, at least 130 g/100 g or at least 150 g/100 g. This has been found to further improve the low temperature solubility of the salts of the invention.

Particularly preferred embodiments of the present invention are thus where the lubricity enhancer comprises a salt formed by the reaction of:

Tall oil fatty acid with di-n-butylamine,

Tall oil fatty acid with tri-n-butylamine,

Rapeseed oil fatty acid with di-n-butylamine,

Rapeseed oil fatty acid with tri-n-butylamine,

Soy bean fatty acid with di-n-butylamine,

Soy bean fatty acid with tri-n-butylamine,

Sunflower oil fatty acid with di-n-butylamine, and

Sunflower oil fatty acid with tri-n-butylamine.

Preferably, the salt has a flash point of at least 50° C., more preferably at least 60° C.

The salt may conveniently be produced by mixing the carboxylic acid with the amine. The order in which one component is added to the other is not important. The molar ratio of the amount of acid to the amount of amine is suitably from 10:1 to 1 :10, preferably from 10:1 to 1:2, more preferably from 2:1 to 1:2, for example, around 1:1. In an embodiment, a molar ratio of 1.1:1 to 1:1.1 has been found to be suitable. The reaction may be conducted at room temperature, but is preferably heated gently, for example to 40° C.

Preferably, the solvent is a hydrocarbon solvent, such as an aromatic hydrocarbon solvent. Examples of hydrocarbon solvents include petroleum fractions such as naphtha, kerosene, diesel and heater oil; aromatic hydrocarbons such as aromatic fractions, e.g. those sold under the ‘SOLVESSO’ tradename; alcohols and/or esters; and paraffinic hydrocarbons such as hexane and pentane and isoparaffins. The additive concentrate may also contain further additives as required. Such further additives are known in the art and include, for example the following: detergents, antioxidants (to avoid fuel degradation), corrosion inhibitors, dehazers, demulsifiers, metal deactivators, antifoaming agents, cetane improvers, co-solvents, package compatibilisers, reodourants, additives to improve the regeneration of particulate traps, middle distillate cold flow improvers and other lubricity additives.

In accordance with a second aspect, the present invention provides a fuel oil composition comprising a major amount of a fuel oil and a minor amount of an additive concentrate according to the first aspect.

Preferably, the oil is a fuel oil, e.g., a petroleum-based fuel oil, especially a middle distillate fuel oil. Such distillate fuel oils generally boil within the range of from 110° C. to 500° C., e.g. 150° C. to 400° C. The fuel oil may comprise atmospheric distillate or vacuum distillate, cracked gas oil, or a blend in any proportion of straight run and thermally and/or refinery streams such as catalytically cracked and hydro-cracked distillates. The most common petroleum distillate fuels are kerosene, jet fuels, diesel fuels, heating oils and heavy fuel oils. The heating oil may be a straight atmospheric distillate, or it may contain minor amounts, e.g. up to 35 wt %, of vacuum gas oil or cracked gas oil or of both.

Other examples of fuel oils include Fischer-Tropsch fuels. Fischer-Tropsch fuels, also known as FT fuels, include those described as gas-to-liquid (GTL) fuels, biomass-to-liquid (BTL) fuels and coal conversion fuels. To make such fuels, syngas (CO +H₂) is first generated and then converted to normal paraffins by a Fischer-Tropsch process. The normal paraffins may then be modified by processes such as catalytic cracking/reforming or isomerisation, hydrocracking and hydroisomerisation to yield a variety of hydrocarbons such as iso-paraffins, cyclo-paraffins and aromatic compounds. The resulting FT fuel can be used as such or in combination with other fuel components and fuel types such as those mentioned in this specification. Also suitable are fuels derived from plant or animal sources such as FAME. These may be used alone or in combination with other types of fuel.

Preferably, the fuel oil has a sulphur content of at most 0.05% by weight, more preferably of at most 0.035% by weight, especially of at most 0.015%. Fuels with even lower levels of sulphur are also suitable such as, fuels with less than 50 ppm sulphur by weight, preferably less than 20 ppm, for example 10 ppm or less.

Preferably, the amount of additive concentrate used is such that the salt is present in the fuel oil at a level of between 5 and 1000 ppm by weight, more preferably between 10 and 500 ppm, even more preferably between 10 and 250 ppm, especially between 10 and 150 ppm, for example, between 50 and 150 ppm. In accordance with a third aspect, the present invention provides the use of an additive concentrate according to the first aspect to improve the lubricity of a fuel oil having a sulphur content of less than 0.05% by weight.

In accordance with a fourth aspect, the present invention provides a method of storing an additive concentrate comprising a lubricity enhancer at a temperature of −30° C. such that the concentrate remains clear and bright for at least 14 days, the method comprising dissolving 10 to 90% by weight of a salt formed by the reaction of a carboxylic acid with di-n-butylamine or tri-n-butylamine, in a hydrocarbon solvent.

The additive of the invention may contain a single salt or a mixture of more than one salt. It may also be used in combination with one or more co-additives

The invention will now be described by way of example only.

EXAMPLE 1

Tall oil fatty acid, with a saturate content of ca. 2% and a rosin acid content of ca. 1.8%, (TOFA-1) (50.0 g, 173 mmoles) was added to a beaker with stirring. Di-n-butylamine (22.36 g, 173 mmoles) was then added to the beaker. An exotherm of ca. 38.3° C. was measured indicating that the two components reacted. FTIR analysis of the reaction product showed a reduction in the strong carboxylic acid peak at 1710 cm⁻¹ compared to the starting acid, and a corresponding appearance of carboxylate antisymmetric and symmetric stretches at 1553 and 1399 cm⁻¹ as well as the appearance of a broad range of peaks 2300-2600 cm⁻¹ assignable to ammonium species. This was a clear indication of the formation of a salt. The flash-point of the reaction product was 67° C.

EXAMPLE 2

Example 1 was repeated using tri-n-butylamine in place of di-n-butylamine. Tall oil fatty acid, with a saturate content of ca. 2% and a rosin acid content of ca. 1.8%, (TOFA-1) (30.0 g, 94.5 mmoles) was added to a beaker and with stirring. Tri-n-butylamine (17.52 g, 94.5 mmoles) was then added to the beaker. An exotherm of ca. 13° C. was measured indicating that the two components reacted. FTIR analysis of the reaction product showed a reduction in the strong carboxylic acid peak at 1710 cm⁻¹ compared to the starting acid, and a corresponding appearance of carboxylate antisymmetric and symmetric stretches at 1571 and 1371 cm⁻¹ as well as the appearance of a broad range of peaks 2300-2600cm⁻¹ assignable to ammonium species. This was a clear indication of the formation of a salt. The flash-point of the reaction product was 90° C.

EXAMPLE 3

Example 1 was repeated using a Tall oil fatty acid with a saturate content of ca. 2% and a rosin acid content of ca. 0.8%, (TOFA-2).

EXAMPLE 4

Example 2 was repeated using a Tall oil fatty acid with a saturate content of ca. 2% and a rosin acid content of ca. 0.8%, (TOFA-2).

EXAMPLE 5

Rape seed oil fatty acid (33.87 g) was added to solvent (Atasol 150, 50 g) at ca. 30° C. and agitated until well mixed. Di-n-butylamine (16.13 g) was then charged to the mixture at ambient temperature. An exotherm was observed indicating that the two components reacted. After the exotherm had diminished, agitation was continued for a further four hours. The product was pumped out below the flash point of the solvent.

EXAMPLE 6 (COMPARATIVE)

Example 1 was repeated using di-n-ethylamine in place of di-n-butylamine. The flash point of the product obtained was 42° C.

EXAMPLE 7 (COMPARATIVE)

Example 3 was repeated using di-n-ethylamine in place of di-n-butylamine. The flash point of the product obtained was 42° C.

EXAMPLE 8 (COMPARATIVE)

Example 1 was repeated using di-n-hexylamine in place of di-n-butylamine. The flash point of the product obtained was 122° C.

EXAMPLE 9 (COMPARATIVE)

Example 3 was repeated using di-n-hexylamine in place of di-n-butylamine. The flash point of the product obtained was 122° C.

Stability Testing

Additive concentrates of the salts of Examples 1-4 were made up as 33, 50 and 75 wt % solutions in Solvesso 100 and tested for low temperature stability at −30° C. and −40° C. alongside similar solutions of Comparative Examples 5-8. The results are given in Table 1 below. In the table, the data refer to the number of days that the test sample remained clear and bright. All tests were run for a maximum of 28 days, so a result of ‘28’ in the Table does not necessarily indicate a failure on day 28. It is clear from the results obtained that concentrates according to the invention show improved stability at high concentration and low temperature compared to those containing salts produced from either longer or shorter chain length, linear amines.

EXAMPLE 10 (COMPARATIVE)

As a further comparison, salts were produced by reacting n-butylamine with TOFA-1 and TOFA-2 following the same procedure as Examples 1 and 3. Concentrates containing 50 wt % in Solvesso 100 showed only very limited stability at −40° C. (3 days for the TOFA-1 salt). Concentrates containing 75 wt % in Solvesso 100 showed no stability at all at −40° C. TABLE 1 Additive concentration/ Days stable Days stable wt % Example at −30° C. at −40° C. 33 1 28 28 2 28 28 3 28 28 4 28 28 6 28 7 7 3 1 8 28 28 9 28 28 50 1 28 28 2 28 28 3 28 14 4 28 28 6 7 0 7 1 0 8 28 0 9 28 0 75 1 14 0 2 28 28 3 3 0 4 28 28 6 0 0 7 0 0 8 0 0 9 0 0 HFRR Testing

The salts prepared in Examples 1-4 above were tested in two low-sulphur diesel fuels (details given in Table 2) using the High Frequency Reciprocating Rig 5 (HFRR) test in accordance with BS EN ISO 12156-1 (2000). Results are given in Table 3. The HFRR value for untreated Fuel 1 was 664 μAm, and that for untreated Fuel 2 was 518 μAm. TABLE 2 Unit Fuel 1 Fuel 2 Specification Density kg/m³ 811.1 858.4 Kv (40° C.) cSt 1.942 2.883 Kv (20° C.) cSt 2.843 4.597 Cetane No. 58.1 41.9 Sulphur wt % <0.0005 0.0428 Distillation characteristics IBP ° C. 175.0 187.3 10% ° C. 206.1 219.2 50% ° C. 235.2 270.4 95% ° C. 279.1 333.6 FBP ° C. 291.8 347.3

TABLE 3 Treat HFRR in Fuel HFRR in Fuel Example rate/ppm 1/μm 2/μm 1 50 646 385 100 469 377 150 438 — 2 50 648 — 100 608 — 150 522 — 200 433 — 3 50 666 425 100 451 329 4 50 654 — 100 614 — 150 525 — 200 477 — 250 414 — 300 400 —

It is clear from the results shown in Table 3 that the additive concentrates of the invention are effective lubricity enhancers for low sulphur content diesel fuels.

The salt produced in Example 5 was tested in a number of diesel fuels. The material was used as a 50% solution in Solvesso 150 and added to the diesel fuels at a treat rate 100 wppm (calculated based on active ingredient content). HFRR test results are given in Table 4 below. Each value in the Table is the average of a number of test. The designation of the fuels used e.g. Korean ULSD(1) and Korean ULSD(2), indicates that two different fuels of the same general type were used. TABLE 4 HFRR HFRR of base (100 wppm of salt Fuel fuel/μm of Example 5)/μm Korean ULSD(1) 612 453 Japanese ULSD 491 414 American ULSD 771 552 US kerosene(1) 703 526 US kerosene(2) 686 437 Mk 1 diesel(1) 617 471 ULSD(1) 684 426 ULSD(2) 603 438 Mk 1 diesel(2) 667 407 Canadian jet fuel 694 497 ULSD(3) 598 410 Korean ULSD(2) 579 424

A good improvement in lubricity was seen for all fuels. Additionally, the 50% solution of the salt of Example 5 in Solvesso 150 was tested for low temperature stability. The sample remained clear and bright when held at −30° C. for 28 days. 

1. An additive concentrate comprising a lubricity enhancer dissolved in a solvent; wherein the lubricity enhancer comprises from 10 to 90% by weight of the concentrate; wherein the lubricity enhancer comprises a salt formed by the reaction of a carboxylic acid with di-n-butylamine or tri-n-butylamine; and wherein the concentrate remains clear and bright for at least 14 days when held at −30° C.
 2. A concentrate according to claim 1, wherein the lubricity enhancer comprises from 33 to 50% by weight of the concentrate, and wherein the concentrate remains clear and bright for at least 14 days when held at −40° C.
 3. A concentrate according to claim 1, wherein the lubricity enhancer comprises from 33 to 50% by weight of the concentrate, and wherein the concentrate remains clear and bright for at least 28 days when held at −30° C.
 4. A concentrate according to claim 1 wherein the carboxylic acid comprises a fatty acid or a mixture of fatty acids.
 5. A concentrate according to claim 4 wherein the carboxylic acid comprises tall oil fatty acid or rape seed oil fatty acid.
 6. A concentrate according to claim 1 wherein the solvent is a hydrocarbon solvent.
 7. A fuel composition comprising a major proportion of a fuel oil and a minor proportion of an additive concentrate comprising a lubricity enhancer dissolved in a solvent; wherein the lubricity enhancer comprises from 10 to 90% by weight of the concentrate; wherein the lubricity enhancer comprises a salt formed by the reaction of a carboxylic acid with di-n-butylamine or tri-n-butylamine; and wherein the concentrate remains clear and bright for at least 14 days when held at −30° C.
 8. A fuel composition according to claim 7, wherein the fuel oil comprises a middle distillate fuel oil having a sulphur content of at most 0.05% by weight.
 9. A concentrate according to claim 7, wherein the lubricity enhancer comprises from 33 to 50% by weight of the concentrate, and wherein the concentrate remains clear and bright for at least 14 days when held at −40° C.
 10. A concentrate according to claim 7, wherein the lubricity enhancer comprises from 33 to 50% by weight of the concentrate, and wherein the concentrate remains clear and bright for at least 28 days when held at −30° C.
 11. A concentrate according to claim 7 wherein the carboxylic acid comprises a fatty acid or a mixture of fatty acids.
 12. A concentrate according to claim 11 wherein the carboxylic acid comprises tall oil fatty acid or rape seed oil fatty acid.
 13. A concentrate according to claim 7 wherein the solvent is a hydrocarbon solvent.
 14. A method for improving the lubricity of a fuel oil having a sulphur content of at most 0.05% by weight comprising: adding an additive concentrate comprising a lubricity enhancer dissolved in a solvent to a fuel oil, wherein the lubricity enhancer comprises a salt formed by the reaction of a carboxylic acid with di-n-butylamine or tri-n-butylamine and is from 10 to 90% by weight of the concentrate, and wherein the concentrate remains clear and bright for at least 14 days when held at −30° C. 